Both paper wet web strength and paper wet strength have always been desired strongly by papermakers. Paper wet web strength refers to the ability of a never-dried paper web on a paper machine to resist breakage. Low wet-web strength can lead to frequent breaks which interrupt production and lower paper machine efficiency. On the other hand, paper wet strength refers to the strength of re-wetted paper. Good wet strength is necessary for many commercial paper products, such as filter papers, sanitary tissues, and packaging papers.
The paper web is mainly a matrix of fibers. The ability of the wet web to resist tearing depends upon both the strength of fiber-fiber bonds and the ability of the fiber network to stretch.1 Since wet webs contain as much as 85% water at the end of the forming section, modern paper machines usually support the web through to the press section where water content is about 50%. Under these conditions capillary forces and mechanical entanglement are the primary contributors to fiber-fiber bonding and wet web strength.2, 3 The standard approaches to improving wet web strength are to decrease the water content or increase the long fiber fraction.4 However, increased costs or lower production rates limit these options. Two polymeric additives, chitosan and cationic aldehyde starch, were proposed to enhance wet web tensile strength by cross-linking fibers.5, 6 Unfortunately, both polymers are impeded at alkaline conditions, which are preferred for the modern papermaking process. Chitosan is water soluble only at acidic condition, while the adhesion of cationic aldehyde starch to fibers is weakened significantly at above neutral pH.7 
A wide range of commercial additives have been applied by papermakers to increase paper wet strength. Under acidic papermaking conditions, urea-formaldehyde resins and melamine-formaldehyde resins are preferred. Whereas, wet-strength resins based on polyamine-epicholorohydrin chemistry are favored under neutral and alkaline conditions.8 It was proposed that upon drying, wet-strength resins cross-link with themself and also form covalent bonds with paper fibers, leading to increased paper wet strength.9 However, most commercial wet-strength resins are not environmentally friendly. At the same time, they are not stable under aqueous conditions and can only be stored for a short period of time. Recently, much research work has focused on developing highly efficient and stable green additives to increase paper wet strength. Examples are polyvinylamine10, polyelectrolyte complexes11, and borate/guar gel12.